There are plenty of reasons to want to perform that kind of scan on the internet. Sometimes, if all you're looking for is a a sample of how many people are running version X of a product versus version Y, you really don't need to map the whole internet.　　You can just sweep for your port of interest across a few million random IPs and call it a day.

Sometimes, though, if you are building a directory, or if you just want a really fantastically beautiful map, you scan the whole shebang.

Doing it all in a few minutes is a little unncessary, though, even if it is neat how much better scanners have been getting lately. A scan that merely takes a few days is plenty fast.